In the long and proud history of flight research at what is now called the NASA Dryden Flight Research Center, the D-558 project holds a special place as being one of the earliest and most productive flight research efforts conducted here. Data from the D-558 and the early X-planes enabled researchers at what became NASA's Langley Research Center to correlate and correct test results from wind tunnels with actual flight values. Then, the combined results of flight and wind-tunnel testing enabled the U.S. aeronautical community to solve many of the problems that occur in the transonic speed range (about 0.8 to 1.2 times the speed of sound), such as pitch-up, buffeting, and other instabilities. This enabled reliable and routine flight of such aircraft as the century series of fighters (F-100, F-102, F-104, etc.) as well as all commercial transport aircraft from the mid-1950s to the present. The Douglas D-558-1 Skystreak and D-558-2 Skyrocket were, with the Bell XS-1, the earliest transonic research aircraft built in this country to gather data so the aviation community could understand what was happening when aircraft approached the speed of sound (roughly 741 miles per hour at sea level in dry air at 32 degrees Fahrenheit). In the early 1940s, fighter (actually, in the terms of the time, pursuit) aircraft like the P-38 Lightning were approaching these speeds in dives and either could not get out of the dives before hitting the ground or were breaking apart from the effects of compressibility-increased density and disturbed airflow as the speed approached that of sound and created shock waves.